Anhydride-grafted polyolefins have been widely used as tie-layers or compatibilizing resins for bonding polar polymers to polyolefin substrates either in coextrusion applications or extrusion-coating applications. In particular, maleic anhydride-grafted polypropylenes and polyethylenes have been popular and useful in bonding polar resins such as ethylene vinyl alcohol copolymers (EVOH), polyamides, ionomers, and polyvinyl alcohol copolymers (PVOH). The use of these polar resins is of interest because they impart high gas barrier properties to the coextruded or coated polyolefin film substrate. These anhydride-grafted polyolefins are used either as discrete tie-layers between the polar polymer and the substrate layer or blended with non-grafted polyolefin homopolymer or copolymer to improve the compatibility of the substrate polyolefin with the polar material. Such polar resins like EVOH materials typically require the use of adhesion promoters and/or tie-layer resins in order for them to bond adequately to polyolefin substrates. Without such tie resins, EVOH materials or related materials like PVOH tend to peel off easily from the polyolefin substrate, resulting in loss of barrier properties and poor appearance.
Accordingly, several known tie-layers exist for EVOH materials. For example, U.S. Pat. No. 4,650,721 discloses a process to improve the otherwise poor bonding of EVOH or PVOH layers in oriented films through the use of tie resins, namely maleic anhydride acid-grafted polyolefins.
U.S. Pat. No. 5,153,074 discloses metallized oriented multilayer films that include EVOH and blends of a maleic anhydride-modified propylene homopolymer or copolymer as the substrate to which the EVOH is contiguously adhered. The EVOH layer is used as a metallizing surface for the vapor deposition of aluminum. Again, the use of an adhesion-promoting material is important. It is known that EVOH is relatively hard to stretch compared to polypropylene. Consequently, only limited grades of EVOH, for example one with 48 mole % of ethylene, can be co-processed with OPP without forming any surface defects. Using lower ethylene mole % EVOH (e.g. 44% or 38%) in biaxial orientation causes surface defects like stress fractures or process issues like film breaks due to the higher crystallinity of the EVOH.
U.S. Pat. No. 5,175,054 discloses the solution-coating of a mixture of solution-grade EVOH or PVOH containing about 80 wt. % of vinyl alcohol and aqueous dispersion-grade of the ionomer of the alkali salt of ethylene-methacrylic acid copolymer. This coating is applied to an oriented polymer substrate and subsequently metallized. The ionomer acts as an adhesion promoter to assure adequate adhesion of the EVOH or PVOH to the polyolefin (polypropylene) substrate which is otherwise poor without the presence of the ionomer.
U.S. Pat. No. 5,688,556 discloses polymeric film structures using EVOH or amorphous nylon layers wherein tie-layer materials like maleic anhydride-modified polypropylenes are employed as adhesion promoters.
U.S. Patent Publication No. 2005-0186414 describes polymeric film structures using EVOH/amorphous nylon blends as barrier layers coextruded with a polyolefin core layer blended with an adhesion-promoting resin. This adhesion-promoting resin is a carboxylic acid-modified polyolefin or an anhydride-grafted polyolefin. World Patent Application WO 02/45958 also describes polymeric film structures employing EVOH and amorphous nylon blends wherein a tie-layer material such as anhydride-grafted polypropylene is required to promote adhesion between the polar blend and the polyolefin substrate.
The prior art uses anhydride-grafted polyolefins as tie-layer resins for bonding polar polymers. One of the limitations of grafting polyolefins with anhydride groups is that the amount of anhydride is usually limited to 1 wt. % or less. Grafting more anhydride than this onto the polyolefin can result in degradation of the polyolefin. This limit on the amount of anhydride grafting can limit the adhesion promoting performance of the tie-layer resin.
The adhesion between a polar and a non-polar layer depends upon their chemistry and physics. Chemically, it depends upon the interaction forces between them. Physically, it depends upon the stiffness and thickness of the layers. To facilitate adequate bonding between polar and non-polar materials, a tie-layer or adhesion promoter is usually utilized. Such tie-layers typically incorporate some degree of polar functionality to promote compatibility with the polar material of interest, yet not too high a degree of polar functionality in order to maintain compatibility with the non-polar material. This can be important during coextrusion processes and subsequent orientation of these cast coextruded products in oriented films. Without using such a tie-layer between for example, EVOH and polypropylene in a coextruded film process, the EVOH layer can separate from the polypropylene basefilm during casting. Also, the EVOH can delaminate and wrap downstream rollers such as machine direction orientation (MDO) processes during machine direction stretching. Using a tie-layer to promote adhesion between the EVOH and polypropylene avoids this problem.
Polyolefins such as polyethylene and polypropylene are often grafted with maleic anhydride to produce polar functionalized polymers and have proven to be effective as a tie-layer or adhesion promoter for bonding non-polar polypropylene or polyethylene to a polar polymer such as EVOH. The amount of grafting can control the effectiveness of the tie-layer, i.e. the higher the amount of anhydride grafting, the more effective it can be as an adhesion promoter for bonding polar materials to non-polar materials. In addition, most anhydride-grafted polyolefins contain rubbers as a tie-layer to enhance the adhesion between polar and non-polar materials. This rubber component of the tie-layer helps improve adhesion by being relatively softer and more flexible and thus conform better to the more crystalline polar (e.g. EVOH) and non-polar (e.g. polypropylene) materials of interest. However, if the rubber content is too high or too much tie-layer resin is used, the resultant film produced is often very hazy and aesthetically less pleasing.
The amount of grafting is limited as such anhydrides can cause degradation of the polyolefin chain it is grafted to. This can usually be seen in a large increase in melt flow rates after grafting. For example, many anhydride-grafted polypropylenes are limited to 0.5 wt. % anhydride (typically less than 1 wt. % anhydride) due to this issue with polymer degradation. At 1 wt. % or more anhydride, the polypropylene melt flow rate increases greatly, melt strength and molecular weight is decreased significantly, to the point where it is unsuitable for oriented film production or casting. In addition, there can be limitations imposed upon the amount of anhydride grafting for food packaging applications due to FDA regulations.
Besides anhydride-grafted polyolefins, ethylene polar terpolymers such as Arkema's LOTADER resins have been used as a tie-layer between polyethylene and polar resins like EVOH. However, versions of these ethylene polar terpolymers have typically shown poor effectiveness as a tie-layer for polypropylene and polar resins and thus, have not been used in oriented polypropylene film applications. Typically, only high co-monomer content (e.g. 16-25 wt. % butyl acrylate or methyl acrylate co-monomer) ethylene polar terpolymers can be effectively used to bond to polypropylene substrates. Unfortunately, the drawbacks of such high co-monomer content ethylene polar terpolymers are that their melting points are much lower than the low content co-monomer versions (77-90° C. vs. 105-107° C., respectively), so the material is much softer (modulus 20-60 MPa vs. 110-120 MPa, respectively) and stickier (Vicat softening point 40-55° C. vs. 65-80° C., respectively). Consequently, in film extrusion and orientation, the use of these high co-monomer content ethylene polar terpolymer materials can result in severe processing problems such as resin pellet bridging in the resin feed system of the extruder throat, sticking and subsequent film breakage of the film due to the cast roll, MDO rolls, and other downstream processes which involve heat for orientation and heat-setting. Thus, the LOTADER materials are not effective for polypropylene film manufacturing because: 1) if low co-monomer content ethylene polar terpolymer is used, poor adhesion is found to the polypropylene; 2) if high co-monomer content ethylene polar terpolymer is used, poor processability results from the extrusion, casting, and orientation processes.
The chemistry and composition of the LOTADER ethylene polar terpolymers are different from anhydride-grafted polyolefins. The former ones are made by polymerization with 3 co-monomers (i.e. ethylene, butyl or methyl acrylate, and maleic anhydride or glycidyl methacrylate), and the polar functional groups are incorporated as part of the polymer chain's backbone. In contrast, the anhydride-grafted polyolefins are made by grafting the functional anhydride groups onto the side chains of a polymer via radical reactions. The degree in which anhydride groups can be grafted onto polypropylene or ethylene-propylene copolymers is limited because this radical reaction used in the grafting process results in a competing reaction: chain scission of the polypropylene backbone at the tertiary carbons. Normally, high molecular weight anhydride-grafted polypropylene or ethylene-propylene copolymer contain less than 1 wt. % of the anhydride groups for this reason. Popular anhydride-grafted resins such as Mitsui's ADMER product line and Dupont's BYNEL product line are of this type. Some low molecular weight anydride-grafted polypropylenes available from Crompton contain more than 1 wt. % of anhydride functional groups, however, the reason they are low molecular weight is due to the degradation caused by the relatively high amount of anhydride grafting. Such low molecular weight/high anhydride content grafted materials have been used as coupling agents; however, they are not suitable for use as a tie-layer in oriented film applications.